UltraJSON up to 5.12.0 ujson.dump memory leak

Summaryinfo

A vulnerability, which was classified as problematic, has been found in UltraJSON up to 5.12.0. This issue affects the function ujson.dump. This manipulation causes memory leak. This vulnerability is registered as CVE-2026-44660. Remote exploitation of the attack is possible. No exploit is available. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability classified as problematic was found in UltraJSON up to 5.12.0. Affected by this vulnerability is the function ujson.dump. The manipulation with an unknown input leads to a memory leak vulnerability. The CWE definition for the vulnerability is CWE-401. The product does not sufficiently track and release allocated memory after it has been used, which slowly consumes remaining memory. As an impact it is known to affect availability. The summary by CVE is:

UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+. Prior to 5.12.1, when ujson.dump() writes to a file-like object and the write operation raises an exception, the serialized JSON string object is not decremented, leaking memory. Each failed write operation leaks the full size of the serialized payload. This vulnerability is fixed in 5.12.1.

It is possible to read the advisory at github.com. This vulnerability is known as CVE-2026-44660 since 05/07/2026. The exploitation appears to be easy. The attack can be launched remotely. The exploitation doesn't need any form of authentication. Technical details of the vulnerability are known, but there is no available exploit. The attack technique deployed by this issue is T1499 according to MITRE ATT&CK.

The vulnerability scanner Nessus provides a plugin with the ID 318086 (Linux Distros Unpatched Vulnerability : CVE-2026-44660), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 5.12.1 eliminates this vulnerability. The upgrade is hosted for download at github.com. Applying the patch 82af1d0ac01d09aa40c887b460d44b9d9f4bccd9 is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the databases at Tenable (318086) and EUVD (EUVD-2026-32663). Be aware that VulDB is the high quality source for vulnerability data.

Productinfo

Name

• UltraJSON

Version

• 5.0
• 5.1
• 5.2
• 5.3
• 5.4
• 5.5
• 5.6
• 5.7
• 5.8
• 5.9
• 5.10
• 5.11
• 5.12.0

License

• open-source

Website

• Product: https://github.com/ultrajson/ultrajson/

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